In recent years, secondary batteries, which can be charged and discharged, have been widely used as an energy source for wireless mobile devices. In addition, the secondary batteries have attracted considerable attention as an energy source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel. Therefore, the secondary batteries are being applied to an increasing number of applications owing to advantages thereof and, in the future, the secondary batteries are expected to be applied to even more applications and products.
As applications and products, to which the secondary batteries are applicable, are increased, kinds of batteries are also increased such that the batteries can provide powers and capacities corresponding to the various applications and products. In addition, there is a strong need to reduce the size and weight of the batteries applied to the corresponding applications and products.
Small-sized mobile devices, such as a mobile phone, a personal digital assistant (PDA), a digital camera, and a laptop computer, use one or several small-sized, lightweight battery cells for each device according to the reduction in size and weight of the corresponding products. On the other hand, middle or large-sized devices, such as an electric bicycle, an electric motorcycle, an electric vehicle, and a hybrid electric vehicle, use a middle or large-sized battery module (a middle or large-sized battery pack) having a plurality of battery cells electrically connected with each other because high power and large capacity are necessary for the middle or large-sized devices.
The size and weight of a battery module is directly related to an accommodation space and power of a corresponding middle or large-sized device. For this reason, manufacturers are trying to manufacture small-sized, lightweight battery modules. Furthermore, for devices, such as an electric bicycle and an electric vehicle, which are subject to a large number of external impacts and vibrations, require stable electrical connection and physical coupling between components constituting the battery module. In addition, a plurality of battery cells is used to accomplish high power and capacity and, therefore, the safety of the battery module is regarded as important.
A cylindrical battery cell, a prismatic battery cell, and a pouch-shaped battery cell, which are classified based on their shapes, are used as a unit cell of such a battery module or battery pack. Especially, the pouch-shaped battery cell, which can be stacked with high integration, has a high energy density per weight, is inexpensive, and can be easily modified, has attracted considerable attention.
FIG. 1 is a perspective view typically showing a conventional representative pouch-shaped battery cell. The pouch-shaped battery cell 10 shown in FIG. 1 is configured to have a structure in which two electrode leads 11 and 12 protrude from the upper and lower ends of a battery body 13, respectively, in a state in which the electrode leads 11 and 12 are opposite to each other. A battery case 14 includes upper and lower case parts. That is, the battery case 14 is a two-unit member. An electrode assembly (not shown) is received in a receiving part which is defined between the upper and lower case parts of the battery case 14. The opposite sides 14b and the upper and lower ends 14a and 14c, which are contact regions of the upper and lower case parts of the battery case 14, are bonded to each other, whereby the battery cell 10 is manufactured. The battery case 14 is configured to have a laminate structure of a resin layer/a metal film layer/a resin layer. Consequently, it is possible to bond the opposite sides 14b and the upper and lower ends 14a and 14c of the upper and lower case parts of the battery case 14, which are in contact with each other, to each other by applying heat and pressure to the opposite sides 14b and the upper and lower ends 14a and 14c of the upper and lower case parts of the battery case 14 so as to weld the resin layers thereof to each other. According to circumstances, the opposite sides 14b and the upper and lower ends 14a and 14c of the upper and lower case parts of the battery case 14 may be bonded to each other using a bonding agent.
However, the mechanical strength of the battery case 14 is low. In order to solve this problem, there has been proposed a method of mounting battery cells (unit cells) in a pack case, such as a cartridge, so as to manufacture a battery module having a stable structure. On the other hand, a device or a vehicle, in which a middle- or large-sized battery module is installed, has a limited installation space. Consequently, in a case in which the size of the battery module is increased due to the use of the pack case, such as the cartridge, the spatial utilization is lowered.
In addition, the electrode assembly constituting the battery cell is mounted in the battery case in a state in which the electrode assembly is surrounded by the battery case but is not fixed by the battery case. For this reason, the electrode assembly may move toward the electrode leads or may be bent due to external impact or vibration. As a result, an internal short circuit may occur in the battery cell or the electrode assembly may protrude through the battery case, which seriously reduces safety of the battery cell.
Furthermore, the electrode assembly constituting the battery cell repeatedly expands and contracts during charge and discharge of the battery cell. As a result, the thermally welded regions of the battery case may be easily separated from each other and, therefore, an electrolyte may leak from the battery case. In addition, the distance in cathodes and anodes of the electrode assembly may be changed during repeated expansion and contraction of the battery cell with the result that internal resistance of the battery cell may be increased or a short circuit may occur in the battery cell, which causes abrupt lowering in performance of the battery cell.
In order to solve the above problem, a structure in which battery cells are covered by cell covers is used. For a battery module having pouch-shaped battery cells mounted in cell covers, however, a gap may be provided between the battery cells and the cell covers due to structural limitations of the battery cells. As a result, the battery cells may not be securely fixed and may move when impact is applied to the battery cells.
Therefore, there is a high necessity for a battery module having higher stability while solving the above problems.